1. Field of the Art
This invention relates to yarn tensioning method and device suitable for use on textile weaving machines, and more particularly to yarn tensioning method and device which can be effectively used especially on textile weaving machines for fabrication of three-dimensional braids such as square braider, magnaweave, three-dimensional braider and the like.
2. Prior Art
In case of a weaving machine which is directed to the fabrication of braid type three-dimensional fabrics as disclosed in Japanese Laid-Open Patent Application H4-41756, namely, in case of a weaving machine which is arranged to produce three-dimensional fabrics by interweaving yarns fed from a number of yarn carriers, which are driven from a carrier drive mechanism to shift the respective positions along predetermined loci of movements in a common carrier moving plane, it is necessary to provide a yarn orientation angle setting means in order to make the orientation angles of the respective yarns substantially uniform by beating actions in a stabilized manner or in order to control the yarn orientation angles arbitrarily.
The inventor of the present invention proposed an effective means for setting such orientation angles in a prior Japanese Patent Application H4-180408. In a weaving process by a braid type three-dimensional textile weaving machine without an orientation angle setting means of this sort, the greater the yarn draw-out rate in each weaving cycle of the textile to be fabricated, the smaller becomes the orientation angle of the textile. If the orientation angle becomes too small, the resulting fabric cannot be regarded as substantially three-dimensional fabric any longer.
On the other hand, in case of the above-mentioned braid type three-dimensional textile weaving machine, the yarn orientation angle in three-dimensional fabric can be increased by the use of a yarn tensioning device as disclosed in Japanese Laid-Open Patent Application H2-178176 or by the use of a yarn tensioning device as disclosed in Japanese Patent Publication H4-47054. However, a greater orientation angle is reflected by a greater yarn draw-out angle from the yarn tensioning device, so that, when a bobbin is moving away from the center of the fabric being woven, it becomes necessary to draw out the yarn by more than an amount to be consumed by the weaving operation. It follows that, when the bobbin is approaching the center of the fabric, this time there arises a necessity for drawing back the yarn toward the bobbin to take up an excess amount of yarn which would remain overfluously after consumption by a weaving operation. In this regard, as will be explained below, the conventional yarn tensioning devices fail to cope with these problems to a sufficient degree.
Illustrated schematically in FIG. 16 is a typical yarn tensioning device which has been well known in the art.
This yarn tensioning device basically includes a bobbin 3 rotatably mounted on a support shaft 2 which is erected at an eccentric position on a carrier, a clutch 4 mounted on the bobbin 3 and provided with clutch claws, and a rocking lever 5 pivotally supported on the carrier 1 for rocking movements about a fulcrum point 6 in a center portion thereof and engageable at a fore end portion thereof with one of the claws of the clutch 4 to block the rotation of the bobbin 3.
The yarn which is unwound from the bobbin 3 is led out through yarn guides 8, 9 and 10, of which the yarn guides 8 and 10 are fixedly located on the carrier 1 while the yarn guide 9 is provided on a slider member 13 which is slidable along a guide shaft 11 under the influence of the tension in the feed yarn against a biasing force of a tension spring 12. When the tension spring 12 is compresed by a predetermined basing force, the slider member 13 is brought into engagement with a contacting member 14 which is connected to one end of the rocking lever 5, thereby releasing the bobbin 3 from the blocking action of the rocking lever 5 against baising action of a return spring 15.
Therefore, when the feed yarn 7 from the bobbin 3 is drawn out, the tension spring 12 is compressed to a certain extent by the force which acts on the yarn, and the slider member 13 is lifted to push the contact member 14, rocking the lever 5 and turning the fore end portion of the lever 5 out of engagement with a claw of the clutch 4. As a result, the bobbin 3 is turned by one pitch of the claws of the clutch 4, lowering the tension in the feed yarn to a slight degree. Therefore, the slider member 13 is returned under the influence of the biasing force of the tension spring 12, and the rocking lever 5 is also returned under the influence of the biasing force of the return spring 15 to block the rotation of the bobbin 3 again by engagement with the clutch 4.
In this connection, it is known in the art that a pendant weight can be employed in substitute for the above-described tension spring 12 to apply a biasing force to the yarn 7 in a similar manner.
The above-described conventional device makes it possible to cope with fabrics involving large yarn draw-out angles, although it relies on the action of the same tensioning mechanism in both yarn draw-out and draw-back phases of operation, applying substantially the same tensioning force on the yarn in the yarn draw-out and draw-back phases of operation. However, the yarn draw-out phase requires a force for disengaging the lever 5 from a claw of the clutch 4 in contrast to the yarn draw-back phase which does not, so that in average there is a tendency of the tension in the feed yarn becoming slightly higher in the yarn draw-out phase than in the yarn draw-back phase of operation.
On the other hand, on a braid type three-dimensional fabric weaving machine, for example, the fabric and yarns are in the relationship as schematically shown in FIG. 17. As seen in this figure, a fabric weaving process involves yarns 7a which are tensioned in a direction of densifying the fabric and yarns 7b (indicated by bald lines) which are tensioned in a direction of undensifying the fabric. The yarns which are tensioned in the densifying direction are the yarns which are drawn out or wound off from the bobbins on outward bobbin movements away from the center of the fabric, while the yarns which are tensioned in the undensifying direction are the yarns which are drawn back or rewound toward the bobbins on inward bobbin movements toward the center of the fabric. Therefore, if arrangements are made to apply different tensioning forces to the yarns in these two different phases of operation, namely, to apply a high tensioning force to the yarns in a draw-out phase and to apply a low tensioning force to the yarn in a draw-back phase, it becomes possible to increase the orientation angle all the more for densification of the fabric utilizing the beating effects of the highly tensioned feed yarns.
However, it is difficult to increase the yarn orientation angle especially in case of a yarn tensioning device of the arrangements as shown in FIG. 16, in which substantially a constant tensioning force is applied to the yarn throughout the yarn draw-out and draw-back phases of a weaving operation.
Besides, from the standpoint of increasing the amount of the yarn winding on the bobbin 3 and suppressing fluctuations in yarn tensioning force as caused by movements of the yarn carrier 1 durig a weaving operation, the yarn guide 10, which serves to withdraw the yarn from the bobbin 3, is most desirably located above a bobbin support shaft which is erected at the center of the carrier 1. However, according to the known yarn tensioning device shown in FIG. 16, the bobbin support shaft 2 is located in an eccentric position relative to the center line of the carrier for the purpose of locating the bobbin and yarn tensioning device in such spaces on the carrier as would keep them out of contact with bobbins and yarn tensioning devices on other approaching carriers, while locating the yarn tensioning device in a position which would not interfere with the operation of replacing the bobbin for yarn replenishment.
The location of the bobbin 3 in an eccentric position as in the above-described prior art arrangement requires to minimize the bobbin diameter in a degree commensurate with its eccentricity despite a corresponding reduction in size of the yarn winding on the bobbin. In addition, the location of the yarn tensioning device at one side of the bobbin ensures easy mounting and dismantling of the bobbin at the time of its replacement. However, since the yarn 7 is withdrawn from a position deviated from the center of the carrier 1, there is no guarantee that a predetermined tensioning force is constantly exerted on the yarn 7 without influenced by the movements of the carrier which is shifted and turned in step with with the progress of a weaving process.
In this regard, it would be the best choice to mount the yarn tensioning device on a bobbin support shaft 2 which is erected at the center of the yarn carrier 1, and to withdraw a yarn 7 from a bobbin 3 through a yarn guide 10 which is located over the bobbin support shaft 2 at the center of the carrier 1. In such a case, however, there arises a necessity for providing suitable means which would facilitate the mounting and dismantling of the bobbin 3 at the time of its replacement. In case of a textile weaving machine which is arranged to weave three-dimensional textiles like 3-D braids by driving a large number of yarn carriers in a carrier shifting plane, the yarn tensioning device on each carrier are required to be smaller in size and diameter, and this makes the bobbin mounting and dismantling operations more difficult.